Modulation of the intestinal barrier adaptive functions in red-eared slider (Trachemys scripta elegans) invading brackish waters.

Globally, the increase in sea levels is leading to salinization of freshwater, which might influence the freshwater organisms such as red-eared slider, Trachemys scripta elegans. The turtle can invade brackish water environments, in which it must deal with elevated salinity in the gastrointestinal tract that could impact the intestinal function. The intestinal barrier provides a front-line of organismal defense against the chemical and biological environmental insults. In this study, the adaptive functions of the intestinal barrier including intestinal histomorphology, genes involved in intestinal barrier functions, and the intestinal micro-ecosystem were analyzed in the turtles exposed to freshwater (S0), 5‰ salinity (S5) and 15‰ salinity (S15) water for 30 days. The results showed that the intestine of T. s. elegans maintained normal histomorphological structure in the S5 group, whereas the villus height, crypt depth and the number of goblet cells in the S15 group were lower than that in the S5 and S0 groups. In addition, the relative expression levels of epithelial tight junction-related genes and intestinal immune-related genes in the gut were significantly upregulated in the S15 group, compared to the freshwater group. Mucin-2 gene expression was downregulated, but mucin-1 transcript levels were upregulated in salinity-treated groups. Furthermore, the abundances of phylum Proteobacteria, and genera Morganella and Aeromonas in the intestine were particularly enhanced in the S15 group than the S0 and S5 groups. Taken together, these results indicate that the intestinal barrier plays a protective role in T. s. elegans adaptation to brackish water environments. Our results provide a perspective on the evolution of salinity tolerance and help to evaluate the potential danger of the turtle to other species, and understand the challenges that other species must meet with rising sea levels.